(a) Field of the Invention
The present invention relates to a process of producing palm oil or the like. More particularly, the invention is directed to a continuous milling process for extracting palm oil, or similar vegetable edible oil, and in particular relates to the continuous sterilization, stripping, and pressurized digestion of fruit bunches during the palm oil extraction stages.
(b) Description of Prior Art
Oil Palm (Elaeis Guineensis Jacq.), which originated in Africa, exists in wild, semi-wild and cultivated land areas of the equatorial tropics of Africa, South East Asia and South America. At the present time, the oil palm is cultivated over a wide range of tropical climatic conditions and soil types.
For optimum yield, a rain fall of 80 inches or more per year is required, and should be uniformly distributed throughout the year. In addition, the minimum temperature should be within a range of 22.degree. C. to 24.degree. C. and the maximum temperature should be within the range of 29.degree. C. to 32.degree. C. with a daily sunshine exceeding 5 hours per day.
Under the above conditions, it is possible to produce 10 to 12 tons of bunches per acre per year, resulting in a higher yield of palm oil. At the plantation nursery, the seeds (nuts) are allowed to germinate and to grow into young plants before the latter are transplanted into a field. The rate of growth is 1 to 2 feet per year with a life expectancy of 100 years. It is possible to cut the plants and replant them after 25 years as they become taller. As the palm grows, new leaves (fronds) are produced and increase to 30 to 40 in 5 or 6 years, after which the new growth declines to 20-25 per annum.
In the axle of each leaf is a bud which will develop into male and female inflorescences. Male and female flowers are borne in the same palm but on separate inflorescences. The male inflorescence may contain 700 to 1200 flowers, however the female inflorescence may contain several thousands flowers. After pollination the female inflorescence develops into a fruit bunch and it takes 51/2 to 6 months to develop into a ripe fruit bunch.
Most of the oil in the fruit is produced during the last two to four weeks before it reaches full ripeness. The individual fruits in a bunch reach full ripeness over a period of about 2 weeks, with the most exposed fruits ripening first. A mature palm tree can produce 5-10 bunches/year each containing 1000-2000 packed fruits and sometimes more than 2000 fruits in exceptional cases.
Among the plants producing edible oil, the oil palm yields the most oil per hectare. With the recent introduction of African weevil Eloeidobiuss Kamerancius in 1981, especially in Malaysia, the yield has increased greatly. This insect is a very efficient pollinator for oil palm flower and results in bunches with fruitlets in many layers. A typical palm fruit is a drupe, oval in shape, and contains a kernel, which is the true seed. The kernel is surrounded by the fruit wall (pericarp) made up of the hard shell (endocarp), oil bearing tissues (mesocarp) and the skin (exocarp). The palm oil is extracted from the mesocarp of the fruit wall, while the kernel oil is derived from the seed. For the purpose of the present invention, which is concerned with a milling process, the nut (seed) will be defined as a shell with the enclosed seed after the removal of the mesocarp. The pericarp will be understood to mean the mesocarp and exocarp combined. The palm fruit can be classified into three generic types namely Dura, Pisifera and Tenera. The Dura fruit is characterized by its thick shell while the Pisifera fruit has no shell. The hybrid between the two, the Tenera fruit, has a thin shell. ,
The fresh fruit contains enzymes capable of splitting the triglyceride contained in palm oil, into free fatty acid. When the bunch is cut, the enzymes start to catalyse and break down the oil into free fatty acid and partial glyceride. When the enzyme is in contact with the oil, the reaction is rapid. Thus, when the fruit is damaged, the oil released will be in contact with the enzyme and accordingly the free fatty acid will increase.
Palm fruits have to be harvested and transported to the milling factory before the palm oil can be extracted. The theoretical amount of oil contained in the fruit is fixed at the moment when the fruit bunch is cut. The amount of oil contained in the fruit will deteriorate due to free fatty acid increase through bruising and damaging of the fruit in the course of harvesting and transportation to the mill and through aging before the fruits are processed. It is believed that the free fatty acid increase is due to the action of enzymatic endegenous lipase in ripe mesocarp.
Individual fruits do not ripen at the same time and the ripening process is repeated in period of 15 days or less. When the fruit is ripe, it is easily detached to become a loose fruit. For the maximum oil production in the fruits, the fruits have to be cut at the optimum ripeness and cycle time.
Normally, during the final week of the ripening, the oil production will increase. Once the fruit is cut or detached, the oil production stops. If the fruit is left overripe, the fruit will be detached from the bunches and fall to the ground resulting in more loose fruits with the result that the free fatty acid will increase in loose fruits because of bruising. The criterion used to harvest the bunch is based on the count of number of loose fruit fallen on the ground. There is a compromise between the increase of acceptable free fatty acid due to the overripening and the maximum oil produced in the bunches. Optimum harvesting cycle time and optimum minimum ripeness standard are established to ensure a maximum oil content and a minimum acceptable level of free fatty acid in fruit bunches.
By the time the fresh fruit bunch arrives at the factory, it should be processed right away without further delay to prevent any increase in free fatty acid. The oil content in the palm fruit is fixed and the purpose of milling is to ensure a maximum oil extraction while minimizing the losses.
The palm oil milling process is a very unsteady process which depends on a lot of human factors during the harvesting and transportation phases, and also on factors such as types of crops, harvesting cycle, peak crop season, etc.
The prior art process for the extraction of palm oil uses the technology developed some 30 years ago in Africa. The palm oil milling process known in the art comprises the major steps of: preparing fresh fruit bunches (FFB), digestion, oil extraction, oil clarification, sludge separation and kernel separation. There are two main products derived from the palm oil milling process namely palm oil and kernels. In the step involving the preparation of fresh fruit bunches, the process used is batch wise and involves a lot of manual handling for operating equipments such as cages, valves, etc., thereby causing potential safety hazard, inefficient operation resulting in a lot of wastage, poor quality of oil and pollution problems, etc.
For the oil extraction stages, there are several different extraction methods which are known in the art. They are generally classified as the wet process, using a wash liquid to free the oil in palm fruit and the dry process such as one involving the use of a batch type hydraulic press, a semi-continuous type hydraulic press and a continuous type screw press, etc. Each method has its advantages and disadvantages. The wet process ensures that there is no nut breakage but results in the loss of large cell debris in the oil. Thus, the continuous screw press gives a high throughput, and operates at comparatively low energy cost, but tends to result in a high amount of nut breakage when the operating conditions are not ideal.
It will also be realized that the preparation of the fresh fruit bunches is an important step of the whole palm oil milling process, and that it has a great effect on the subsequent operations with regards to the yield and the quality of the product.
As understood from the prior art, the major components of palm fruits are the oil bearing tissue, called the mesocarp and the nut which contains the kernel. In the palm oil milling process, the two major products obtained include the crude palm oil and the kernel. The crude palm oil is extracted from the palm fruit using one of the extraction methods described above while the kernel is obtained by separation from the nut by a cracking process. Other by-products such as fibers, shells and empty bunches are also produced.
For the purpose of palm oil milling, as much as possible of the oil content present in the palm fruit must be extracted, while at the same time minimizing all oil losses. As presently practiced in the art, when the lorry of fresh fruit bunches arrives at the mill, the lorry load is emptied and fresh fruit bunch is piled up in the yard and loading ramp waiting to be processed. Together with the fresh fruit bunches there are loose fruits lying together with the bunches.
The quantity of loose fruits, normally amounts to 10%-20% of the bunches and depends on the harvesting conditions. Most of the loose fruits are damaged and contaminated with sand and dirt, and when in contact with the ground they form an ideal situation for mold growth and for causing an increase of free fatty acid due to enzymatic action. The loose fruits are not separated and they are processed together with the bunches causing oil lose to the bunch stalk through contact and also contaminating the bunches with sand and dirt. The bunches are not processed right away, causing further free fatty acid increase due to mold growth and aging No bunches should normally be allowed to ripen optimally for more than 31/2 days. If the bunches are harvested more than 31/2 days than the optimum and because of a further delay in processing the fresh fruit bunches at the mill, the free fatty acid increase will be aggravated.
From the loading ramp, the fresh fruit bunches are loaded in cages and are transported by operators, normally by rail to the sterilizer. The cages are perforated to allow steam to penetrate into the bunches. The cages are mostly constructed of iron. Due to the contaminating action of vapor in the sterilization vessel, the cages are corroded The corrosion of the cages contribute for a major part to the iron contamination in palm oil. Iron is an oxidant and it accelerates the process of oxidation of palm oil, causing bleachability problem, etc. The cages loaded with fresh fruit bunches are sterilized in the sterilization vessel. The cages generally hold 2.5 tons to 3 tons of fresh fruit bunches.
It is believed that the sterilization results in: (i) deactivation of the oil-splitting enzymes in order to prevent an increase in free fatty acid; (ii) loosening the fruit in the bunch to facilitate the stripping process; (iii) softening the fruit pulp for easier further treatment (digestion) of the fruit; (iv) heating and partially dehydrating the nuts in order that the nuts may be cracked more readily; (v) coagulation of protein in the oil bearing cells to prevent formation of colloidal complexes, thus facilitating the separation/clarification of the oil in the oil recovery process; (vi) hydrolysis/decomposition of mucilaginous material which will facilitates the oil clarification process.
Great care is taken during sterilization to exclude air which interferes with the sterilization efficiency and may cause oxidation of the oil.
The sterilization is normally carried out in a horizontal vessel holding between three (3) to nine (9) cages of fresh fruit bunches. Saturated steam at 40 psig is used as the heating medium. The sterilization is a batch process which consists of the following sequence of operations: heating, venting, deaeration, condensate removal. Single, double or triple peak sterilization is normally practiced.
The total cycle time may vary from 70 to 90 minutes. The physical design of the sterilization vessel entry and outlet.
After sterilization, the cages loaded with sterilized fresh fruit bunches are manually removed from the sterilizer, they are then lifted by means of an overhead crane and emptied into a stripper. The purpose of stripping is to separate the fruitlets and calyx leaves from the branch stalks. The type of stripping machine generally used is a rotating drum made of bars spaced just enough to permit the escape of the fruit and of the calyx leaves. As the drum rotates, the bunches inside the cage are lifted up then dropped back again. Consequently, by this action, the fruits are knocked out of the bunch, while the empty stripped bunches are discharged for disposal.
It will be realized that the prior art process of sterilization and stripping is carried out in a batch wise manner. There are numerous shortcomings and disadvantages. In the sterilization process described above, because the bunches are stacked in cages, steam does not penetrate uniformly and the bunches in the middle of the cages tend to receive less steam treatment. Other disadvantages include the following.
Because of the batch operations, the air in the vessel must be expelled before steam can penetrate , through the bunches. This is normally done in the prior art by steaming and venting the steam, thus resulting in a lot of steam wastage.
The fruit bunches which are in the cages are of different sizes, which means that the time required for the bunches to reach a certain temperature normally depends on the individual bunches. Owing to the fact that the bunches are loaded in cages and the difficulty for steam to penetrate to the center of bunches, it is quite possible that the bunches in the center of the cage will not reach the required temperature.
In addition, due to rigorous steaming and exhaust operations, it is quite possible that there will be a loss of oil from the bunch, due to the fact that the oil is carried away in the steam condensate and exhaust. The condensate which is acidic is responsible for the corrosion of the cages, while the iron contaminates the oil during the milling process.
In the prior art sterilization process, the cycle is fixed and is very seldomly changed. Owing to the different type of crop and different conditions of ripeness, the uneven steaming of the bunches and the fixed time cycle employed in the sterilization often results in hard bunches which are not conducive to the production of palm oil. Also it will affect the subsequent stripping operation and nut cracking. In order to ensure good nut cracking in the kernel plant, the nut has to be heated and conditioned at the later stage.
On the other hand, the sterilization at a fixed temperature may lead to an over sterilization of some overripe bunches. As a result, the kernel tends to become discolored. The tendency of the kernel to become discolored is often used as a criterion to determine the maximum of the sterilization temperature.
The batch sterilization process often produces a boiler upset when the demand of steam is great especially during the steaming cycle. Even though the problems can be overcome by better energy management, the inherent nature of the boiler instability is always here.
In the prior art process, the stripper operates in a batchwise manner. The stripping operation is dependent on the frequency at which the operator dumps the sterilized cages of fruit bunches. The feeding of the sterilized fruit bunches to the stripper is nearly always uneven (over dumping), and the fruit bunches are piled up. This results in oil losses through its absorption in the stalks and the calyx leaves. The calyx fragments have an effect of reducing the oil loss in the cake that is ejected from the press.
Due to the deficiency in the operation of the stripper of the prior art, the latter will produce bad bunches. The stripability also depends on the good performance during the step of sterilization. The presently known stripper has a feeder which is sloped toward the discharge end which will aggravate the piling up of fruit bunches. The loose fruits which are detached from the bunch during the reception at the beginning of the process are stripped together with the rest of the bunch, and it is therefore a waste operation to pass them together with the bunches.
Finally, the present prior art process poses a safety hazard during the lifting of the cages of sterilized fruit bunches prior to the dumping thereof into the stripper.
According to the process of the prior art, after the fruitlets have been stripped, they are sent to the digester. The digester is a cylindrical vessel fitted with vertical rotating shaft carrying a number of stirring arms. The purpose of the digestion is to rupture the oil bearing cells so that the palm oil can be released during the pressing stage. Typical retention time is 30 minutes and the temperature is maintained at 95 to 100 degrees C. The temperature is maintained by jacketed steam, but sometimes live steam is added.
In principle, during the digestion process, two actions occur. Firstly, due to the weakening of cell membrane there is an intensive release of virgin crude oil corresponding to 15%-20% of the weight of the bunches, drawn off through the perforation of the digester and secondly, there is a rupture of oil cell due to stirring action on the fruit mass. In order to ensure complete digestion sufficient retention time is maintained. The digested mass of palm fruit will then be discharged from the digester to the screw press for oil extraction.
The oil cells are bonded to each other and the skeleton of fibers runs lengthwise by inter-cellular bonding. The bonding is pectic in nature. The amount of pectin increases during ripening and is soluble in very hot water but not in cold water. When the bond dissolves, it disintegrates into oil cells and fibrous materials.
The wall of the oil cell is extremely elastic and there is very little difference between the pressure inside and outside the cell. The collapse of the cell wall requires extremely high pressure, even though, it is not probable that all the cells will be ruptured simultaneously. During the process of sterilization, the effect of steam will help the cell wall to collapse (not rupture) and it is believed that this it is achieved partly by hydrolysis and partly by coagulation.
In the prior art process, the crude oil drained from the digester contains more non-oil solids and cell debris. This oil is manually mixed with the oil coming from the press before going to the clarification process. The cell debris consist of broken unruptured oil cells, which are fairly difficult to recover in the clarification stage. The cell debris will be carried away with the sludge and constitute a source of oil loss. This oil loss represents close to 1% of the production of oil.
In the prior art process, because the oil drained from the digester contains more fibrous materials (non-oil solids), this loss will result in lower percentage of fiber in the press cake produced in the press, which will lead to an extra nut breakage in the press and higher oil loss in the pressed fiber. A certain nuts/pericarp ratio of about 35-40/65-60 should be maintained for proper operations in the press.
In prior art process, because the sterilization does not give sufficient treatment when processing different type of crops and their ripeness, and because fixed time cycle is employed as the result, the time required for a proper digestion needs to be optimized.
Because the prior art is concerned with a batch process during sterilization and stripping, the operation of the digester tends to be easily upset if there is a break in the process during the sterilization step. The last stage of the digester is never operated at full level, thus reducing the retention time of the fruitlets.
It is an object of the present invention to provide an improved palm oil milling process which eliminates the cages and the handling operations during the sterilization and stripping steps.
It is a further object of the present invention to provide an improved palm oil milling process whereby fresh fruit bunches are processed without delay on reception in the receiving bin while pre-steaming eliminates the chances of an increase in free fatty acid while the fresh fruit bunches laying on the ground are waiting to be processed.
It is another object of the present invention to provide an improved palm oil milling process wherein the conventional batch wise process is converted to a continuous process thus rendering the automation of the entire plant possible.
It is a further object of the present invention to provide an improved palm oil milling process by combining the sterilization and stripping into a single continuous operation thus reducing the time cycle required during a batch-wise conventional sterilization process and optimizing the sterilization/stripping operation in an energy efficiency manner.
It is a further object of the present invention to eliminate the instability of the boiler operations, and to reduce air pollution caused by black smoke during boiler upset by continuously supplying steam to the sterilizer/stripper.
It is a further object of the present invention to provide an improved palm oil milling process which minimizes the loss of oil experienced in the sterilization process of the prior art and improves the bleachability of the oil by eliminating the cages responsible for iron contamination due to rusting.
It is a further object of the present invention to provide an improved palm oil milling process to increase the oil extraction efficiency by treating the stripped fruitlets in a pressurized digester which facilitates the breakage of oil cells and improves the efficiency of oil extraction during the pressing stages, and oil quality and minimizes nut breakage.
It is another object of the invention, to provide a process wherein, upon reception, the fresh fruit bunches will be conveyed to a receiving bin where presteaming will be carried out at the minimum temperature of 55.degree. C. in order to deactivate the enzymatic reaction and to stop the increase of free fatty acids prior to full sterilization/stripping stage.
It is another object of the present invention to provide a continuous process which can be adapted for the conversion of existing palm oil milling processes which rely on a batch process including fresh fruit bunches preparation and screw press for extracting the oil .
It is another object of the present invention to separate the loose fruits for prewash and mild steam treatment in order to eliminate sand and dirt and also to prevent any mold growth in loose fruits.
It is another object of the present invention to provide a process wherein stripped fruitlets may be continuously conveyed by means of an elevator to a surge bin and to regulate the flow of fruitlets to the digester on a continuous basis.
It is yet another object of the preset invention, to minimize oil loss through cell debris in the sludge by providing pressurized digestion which will ensure maximum cell rupture in the digested fruitlets before the pressing stage.